The present invention relates generally to new methods of semiconductor device fabrication, and more particularly to methods to fabricate high-contrast distributed grating structures and semiconductor devices comprising such structures.
Wavelength-selective filters and mirrors are an integral component in photonic integrated circuits and other photonic devices. Applications include integrated laser sources, waveguide couplers, wavelength tunable lasers, resonant filters, modulators, and wavelength-division-multiplexing systems. Such wavelength-selective components and devices are often implemented through the action of a distributed grating structure.
A distributed grating structure is typically a planar periodic arrangement of high and low refractive index linear elements (square rods, in the simplest implementation), so positioned as to interact with the optical signal to be processed. This interaction can be direct, or through evanescent wave interactions.
A working optical device will require, in addition to the distributed grating structure, some means of insuring that the optical mode or modes of interest are so directed or confined that they interact with the distributed grating structure in the desired fashion. This means will often include a waveguide, or waveguide-like structure, a choice particularly suited to integrated optics applications. Common implementations will involve a ridge waveguide located on the top or bottom of the distributed grating structure. Other possibilities include structures to produce planar confinement of incident light, and optical elements which direct the light to pass near or to scatter from the distributed grating structure. These choices are made with regard to the device being designed, and do not significantly alter the distributed grating structure, save in detail.
The periodic interaction contributes to constructive interference for the desired action for some frequencies, and to destructive interference for other frequencies. Because the length of the grating can be quite large, such devices can differentiate between optical wavelengths which are differ only slightly. For example, semiconductor lasers having an optical cavity based on distributed grating structures have been reported with a spectral linewidth as small as 25 kHz, while a wavelength range as large as 5% of the lasing wavelength is simultaneously achieved.
Previous fabrication methods will be described in the context of the InAlGaAs III-V semiconductor alloy system, although distributed grating structures have been applied in other systems. There are two main approaches toward such fabrication in the literature. The first approach makes a filled distributed grating, that is, one where both material components of the grating are solids, and those components are in intimate contact at the boundaries between the grating elements.
To form such filled distributed gratings, one applies a layer of AlGaAs at the level where the grating is desired within the device structure. The thickness of the grating is usually the same as that of the AlGaAs layer. A grating structure is then patterned on the surface of the AlGaAs layer, and is transferred to the layer by etching. Following the etching process, a layer of GaAs, or of AlGaAs with a smaller Al content, is epitaxially regrown over the etched grating layer. Epitaxial regrowth is important for most distributed grating applications, to limit the density of structural defects, which can both scatter light and can provide recombination centers for charge carriers within the device.
When additional device structures are fabricated, the result is a device with a planar grating having alternating regions with two refractive indicesxe2x80x94that of the AlGaAs layer, and that of the GaAs filling. These materials are limited to a low reflective index contrast (hereafter called simply a low-contrast grating structure), with the maximum contrast (ratio of higher to lower refractive index) being about 1.15 for all lattice-matched semiconductor material systems. As a result, rather long distributed gratings have to be used in practical optical devices. For example, a length much greater than 100 microns is required for laser mirrorsxe2x80x94a requirement which greatly increases the required footprint of the laser. However, one benefit of the low-contrast filled distributed grating structures is that the interfaces between the individual grating elements and the high refractive index layers are relatively benign with respect to scattering, diffraction, and other mechanisms whereby the optical mode of interest can be disrupted. Since the grating structure is buried, diffraction losses are also quite small.
A more recent approach aims to obtain high-contrast unfilled distributed grating structures by using air as the second grating material. These are often called surface gratings, and are formed by growing a suitable dielectric layer, then defining and etching a pattern of deep open trenches in the surface of that layer. The trenches must be quite deep so that the optical mode of interest does not strongly interact with the trench""s open ends, the result of which would be large diffraction and scattering losses. Alternately, very shallow trenches can be formed. Such structures will have small diffraction and scattering losses, but will also exhibit weak coupling coefficients to the optical modes. As a result, shallow structures are not very effective at reducing device footprints.
Dielectric contrast on the order of 3-5 can be attained with surface gratings fabricated in conventional semiconductors, with larger values being possible if special materials are used. Such high-contrast deep grating structures can be effective for laser cavities in lengths as small as 5 microns or less.
Although there have been several demonstrations of operating optical devices using high-contrast unfilled distributed grating structures, there remain a multitude of practical problems in their adoption for production devices. At a fundamental level, the open upper surface of the grating structure insures large scattering losses and a decrease of optical confinement. Deep grooves are needed to minimize the diffraction losses at the surface. In addition, efficient gratings require a square-wave shape with vertical and very deep sidewalls and squared corners. This shape is extremely difficult to form, requiring electron-beam lithography and a highly anisotropic etching technique.
Further, the optical mode being processed in a device possessing an unfilled surface grating must be coupled into an air gap before it can be further coupled into a waveguide. This requirement adds considerable diffraction loss and structural complexity. Therefore, despite the dramatic improvement in device footprint size which can be obtained using unfilled surface gratings, such devices remain primarily laboratory research tools.
There is a clear need for high-contrast distributed grating structures which are straightforward to fabricate, allow similar reductions in device footprint size as do the unfilled surface gratings, and which minimize the deleterious effects of the imperfect gratings resulting from any potential fabrication processes. The instant invention seeks to address and largely solve this need.
The present invention is of a new class of methods to fabricate embedded (or buried) filled distributed grating structures. These methods involve the lateral oxidation of buried grating elements, thereby dramatically lowering their dielectric constant, and allowing high-contrast buried gratings to be made. Such gratings allow very small and highly efficient wavelength-selected optical devices to be routinely fabricated.